BEIRUT — The Turkish military struck targets inside Syria on Wednesday in the most serious escalation in international tensions since the Syrian revolt erupted 19 months ago.

There was no immediate indication that the cross-border attack was likely to herald a full blown war, but it nonetheless underscored the long-standing danger that the conflict in Syria could draw in neighboring countries and trigger a wider regional conflagration.

The unspecified targets were hit after a shell apparently launched by the Syrian military during battles with rebels crashed into a Turkish border town, killing five members of a Turkish family. The deaths prompted Turkey to act for the first time on repeated threats to retaliate for encroachments on its territory.

The strikes came on a day of bloodshed in Syria in which human rights groups said 190 people were killed. At least 34 of those were the result of a suicide attack in the heart of Aleppo that devastated a major city square in an area controlled by government forces.

Though this was not the first time the Syrian conflict has spilled into Turkey since Syrians rose up against President Bashar al-Assad last year, these were the first Turkish civilians to die. Within hours, the Turkish military struck back, according to a brief statement issued by the office of Turkish Prime Minister Recep Tayyip Erdogan.

A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

“Our armed forces in the border region responded immediately to this abominable attack in line with their rules of engagement; targets were struck through artillery fire against places in Syria identified by radar,” the statement said.

NATO, of which Turkey is a member, met in emergency session at Turkey’s request and issued a strongly worded statement calling the Syrian shelling “a flagrant breach of international law and a clear and present danger to the security of one of its Allies.”

Although NATO pledged to continue to “stand by Turkey,” it proposed no immediate action.

In Washington, the White House also condemned the Syrian shelling and affirmed the United States’ solidarity with Turkey. “We stand with our Turkish ally and are continuing to consult closely on the path forward,” spokesman Tommy Vietor said.

Pentagon spokesman George Little condemned what he called “the depraved behavior of the Syrian regime.”

Secretary of State Hillary Rodham Clinton reiterated the United States’ support for Turkey in a telephone call with Turkish Foreign Minister Ahmet Davotoglu and pledged that the United States would stand by Turkey in any future discussions of the crisis at the United Nations, according to State Department spokesman Victoria Nuland.

Turkey sent a letter to the United Nations calling on the Security Council “to take necessary action to put an end to such acts of aggression.”

Turks, however, have expressed little appetite for a war with Syria, and Syria’s government has no interest in provoking international military intervention in a conflict that it still believes it can win.

“This is a gesture, not a war,” said Henri Barkey, professor of international relations at Lehigh University, referring to the Turkish retaliation.

Erdogan has repeatedly made bombastic statements about Syria, and the first deaths of Turkish civilians left him with little choice but to retaliate, Barkey said.

“The Turks don’t want to go to war,” he said. “They don’t want to do this by themselves, and there is absolutely no support at home for a war.”

But the incident pointed to the dangers of a conflict that has already polarized the region, stirring ancient rivalries and newer enmities.

“This is the latest manifestation of a worsening situation,” said Andrew Tabler of the Washington Institute for Near East Policy. “This is not the only place it is spilling over. It is spilling over into Jordan, into Lebanon. They’re shelling on a daily basis. The difference today is it’s the first time we have a country calling Assad out.”

In a statement issued before the Turkish retaliation, U.N. Secretary General Ban Ki-moon issued an appeal for restraint, warning that the incident demonstrated the risks the violence in Syria poses to the region.

“Syria’s conflict is threatening not only the security of the Syrian people but increasingly causing harm to its neighbors,” he said.

The strike followed more than a year of growing hostility between Turkey and Syria, former allies who have turned into bitter foes since Erdogan joined his Western allies in calls for Assad to step down. The tensions soared sharply after Syria shot down a Turkish jet in June, killing two pilots over international waters, according to Turkey, and within Syrian territorial waters, according to Syria.

Meanwhile, a surge in the levels of violence in Syria over the summer months has propelled tens of thousands of refugees into Turkey, straining government resources and heightening government frustrations with the failure of its Western allies to take tougher action against Assad.

The number is likely to grow as the violence continues to rage. The suicide attack in Aleppo signaled that the rebels are growing more sophisticated in their strikes against government targets, at a time when the regime is stepping up its assaults against rebel-held areas with aerial bombardments.

According to state media, two suicide bombers detonated car bombs containing more than 2,000 pounds of explosives in the city’s central Saadallah al-Jabri square shortly after 8 a.m., killing 34 people and injuring 122. A third explosion nearby occurred while officers were dismantling a bomb, according to the official Syrian Arab News Agency.

The target, apparently a military officers club, was completely demolished. The blasts also caused widespread damage to the surrounding area, ripping the facades off several buildings. State television showed the bodies of three men wearing military uniforms lying amid piles of debris.

SANA said civilians and military personnel were among those killed, but rebels claimed that all of the victims were members of the security forces, saying that the area represented a legitimate target because it had been turned into what rebel spokesman Mohammed al-Halbi called “a military barracks.” High-ranking officers were living at the officers club, and two government-owned hotels damaged in the blast were being used to accommodate Syrian and Iranian intelligence operatives, he said.

Responsibility for the attack was claimed by the Jabhat al-Nusra, a militant group that has carried out a string of suicide bombings in Damascus and Aleppo in recent months.

Colum Lynch at the United Nations and James Ball, Anne Gearan, Scott Wilson and Craig Whitlock in Washington contributed to this report.

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